Conventionally, in case a scanner and a printer, which can be used individually, are used in combination as a system, condition of the scanner and the printer is displayed to a user in the following manner.
Information of the scanner is displayed on a large-sized liquid crystal display apparatus (hereinafter, just referred to as an LCD), which is provided as a display section of an operation panel of the scanner. On the other hand, information of the printer (such as a message regarding an error) is generally displayed on a display section 1502 of an operation panel 1501 that is provided on a printer main body. FIG. 32 shows an example of such operation panel. The operation panel 1501 is provided with the display section 1502, which is a small-sized LCD, and an operation section 1503, which is composed of various keys 1503a to 1503f. 
However, as shown in FIG. 32, the display section 1502 of the printer main body can display only a limited amount of information, because the display section 1502 is the small-sized LCD that can display only two lines or so.
Hence, the information of the printer is conventionally displayed on the display section, which is the small-sized LCD, where the scanner and the printer are used in combination as a system. Thus, in order to be displayed on such display section, every information of the printer should be as short as two lines. In case of information requiring more than two lines to be fully displayed, scrolling a display thereof by using an up-key and a down-key is necessary to show the display to the user.
Therefore, for example, if the printer is in trouble, such as jamming, available to the user is instructions displayed on the small-sized display section. Because such instructions are hard to follow, it is difficult to deal with the trouble, following the instructions.
Japanese Unexamined Patent Publication, Tokukaihei No. 6-253084 (published on Sep. 9, 1994), discloses a prior art that improves economy of a composite function terminal device having multi functions, such as a printer function, a photocopying function, and a fax function. In order to extend functions of the device, the prior art controls an operation panel built in a standard composite function terminal device, and an operation panel, which is extended display means that can be added or removed from the composite function terminal device, so that both the operation panels are operated in a linked manner.
Moreover, there is a prior art in which a scanner and a printer are integrated to form a unit. Each of the scanner and printer of the prior art cannot be individually used, and detailed information of the printer need to be displayed on a large-sized LCD of the scanner.
However, those prior arts are to be used in the composite machine in which each function is integrated therein, and cannot be employed in the system in which the scanner and the printer, each of which can be individually used, are combined, like the present application.
Furthermore, Japanese Unexamined Patent Publication, Tokukaihei, No. 8-297388 (published on Nov. 12, 1996) discloses an image forming apparatus in which a scanner unit and a printer unit, each of which can be individually used, are combined. The image forming apparatus displays, on a display input apparatus (operation panel) of the scanner unit, information regarding an error or a state of the printer unit.
However, in the image forming apparatus of the publications, displayed on the display input apparatus of the scanner unit is merely information of the condition of the apparatus as a whole, for example, the information of the error or the state of the apparatus, so that the user can grasp the condition of the apparatus as a whole. Therefore, no detailed information of the printer unit, such as the instruction of solving jamming, feeding sheets (paper), or exchanging units, cannot be displayed in a user-friendly form.
Moreover, as to an image forming system in which a scanner and a printer are separately provided so that the scanner and the printer are respectively located in a predetermined position, both the scanner and the printer are generally provided with a user interface section, which includes a display section for displaying various information, and an operation section for giving instructions on operation. Between the two user interface sections, the user mainly operates the interface section that is easier to see and use for him.
Generally in the image forming apparatus in which the scanner and the printer are combined, the user interface section of the scanner is located on such a position (upper position) that the user can easily operate the user interface section, considering that a document is set or exchanged on the image forming apparatus.
However, in case of a usual operation where the user operates the image forming system referring to the information displayed on the display section of the scanner, the user should squat down so as to see and confirm the display section of the printer, which locates underneath the display section of the scanner and cannot be seen without squatting down. Moreover, in the image forming system, display information displayed respectively on the display section of the scanner and that of the printer is shown to the user, when he refers to the operation instructions or confirms the state of the image forming system.
Therefore, a user is, who is unfamiliar with the image forming system, confused. Thus, the user interface section is not user-friendly.
Especially, in case the display section of the scanner and that of the printer have different display capacities, the information displayed on each display section is displayed in a different manner, thereby confusing the user more. Further, while in the image forming system each of the scanner and the printer are individually operated so as to perform different tasks at the same time, it is impossible to operate only the printer without special operational setting by a specific manager, such as a service personnel, when the scanner is detached from the printer, for example, for periodical maintenance or repair of the scanner.
Furthermore, in case a scanner is newly added to the image forming system composed of the printer only, it is impossible to operate the printer and the scanner in combination without a special operational setting by the specific manager such as a service personnel, similarly to the above case.